1. Field of the Invention
The present invention relates to radar systems, and particularly to a system for detecting the presence of aerial power lines or cables in the flight path of an approaching aircraft.
2. Discussion of the Known Art
Elevated power lines pose a serious hazard to aviation. Such structures pose lethal hazards for low-flying aircraft, and have caused the loss of more than 50 lives and $600 million in property damage over a ten year period. Cable strikes have also become the leading cause of helicopter crashes. For example, in 2005, 18 percent of Army helicopter accidents were due to wire strikes.
Methods have been disclosed to reduce the risk of such collisions. One approach is the use of a passive system for sensing the presence of an extremely low frequency (e.g., 60 Hz in the U.S.) electromagnetic (EM) field that exists in the vicinity of live AC power transmission lines. Such systems are limited in range and by ambient weather conditions, and are inherently incapable of detecting power lines that are inactive or temporarily out of service. Power lines may also be configured to minimize the strength of their EM fields in view of health concerns for persons who live, work, or attend school nearby. In such cases, it is even more likely that passive systems will not be adequate to provide ample warning of the power lines to a pilot. See generally, “Assessment of Health Effects from Exposure to Power-Line Frequency Electric and Magnetic Fields”, NIEHS Working Group Report, National Inst. of Health (NIH) Pub. No. 98-3981 (August 1998).
In combat situations, unenergized elevated cables may also be deployed as a countermeasure to obstruct helicopters from approaching a target. A passive cable detection system would therefore be unable to warn a pilot of the presence of such cables. Optical cable detection systems are also known, but their performance is affected by water and dust content in the air, and on surface conditions of target cables.
U.S. Pat. No. 7,136,011 (Nov. 14, 2006) discloses a ground-based field unit for warning of a danger of an impending collision between an aircraft and an elevated obstacle such as an aerial cable structure. The unit includes a mast, a radar antenna inside a casing with an electronics unit to form a radar system for detecting an aircraft in a coverage area, a central processing unit for identifying such aircraft and computing a collision danger area, and a high-intensity light and radio transmitter system that are activated when an aircraft is detected in the danger area. The transmitter provides a radio signal that carries an audible warning, and the system emits a high intensity warning light. The unit may also be arranged to link with and to activate neighboring field units to emit corresponding warnings, according to the patent.
Airborne systems that incorporate radar altimeters for forward obstacle detection, have also been disclosed. See U.S. Pat. No. 6,750,807 (Jun. 15, 2004) and U.S. Pat. No. 6,885,334 (Apr. 26, 2005). The '807 patent relates to a radar altimeter which, in addition to an altimeter antenna, has a forward facing millimeter wave (MMW) antenna arranged for scanning movement. A processor controls a scanning motion of transmissions from the MMW antenna, processes signals received at the altimeter antenna for a portion of the scanning motion, and processes signals received at the MMW antenna for other portions of the scanning motion. The '334 patent discloses the use of a radar altimeter with a forward looking antenna and an electronic terrain or digital elevation map (DEM) to provide precision terrain aided navigation.
Radar scattering cross sections of straight wires, obtained in response to plane electromagnetic waves at different angles of incidence, have been formulated. See, R. F. Harrington, et al, “Straight Wires with Arbitrary Excitation and Loading, IEEE Transactions on Antennas and Propagation”, vol. AP-15, No. 4 (July 1967), at pages 502-15. The radar backscatter characteristics of high voltage power transmission lines, which are typically formed of a number of helically wound aluminum or copper outer strands and steel inner strands, have also been predicted. See, K. Sarabandi, et al, “A Radar Cross-Section Model for Power Lines at Millimeter-Wave Frequencies”, IEEE Transactions on Antennas and Propagation, vol. 51, No. 9 (September 2003), at pages 2353-60. All relevant portions of the two mentioned articles are incorporated by reference.
A need remains, however, for an airborne radar system that can accurately and reliably detect elevated power lines and cables under a variety of environmental conditions and provide ample warning for the pilot to avoid a collision. Such a system is essential if the lives and safety of military and civilian personnel are to be protected when performing low level flight operations, especially under conditions of limited visibility due to time of day, weather, brownout or whiteout conditions, smoke or other battlefield obscurants.